1. Field of the Invention
The present invention relates to optical imaging systems and fabrication techniques therefor. More specifically, the present invention relates to techniques for fabricating holographic projection screens.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
Visual displays are useful for many applications including the simulation of scenery to allow for the training of a vehicle operator. For example, air flight simulators allow a simulator pilot to view imagery on a projection screen while piloting a mock aircraft. As shown in FIG. 1, in a typical simulator, the simulator pilot is positioned at the center of a large diameter dome. The diameter of the dome is typically between 9.5 and 40 feet. The visual display is projected onto the inner surface of the dome by means of one or more projectors located outside the dome. Input imagery is projected through a small hole in the dome onto the hemisphere opposite the point of projection. The dotted line in FIG. 1 shows the boundary of the forward hemisphere into which Projector "A" projects a visual display. The visual display is projected onto the inner surface of the dome (hereinafter the "screen"). The angle of incidence of light at the screen is equal to the projection angle. The angle of reflection of light from the screen is equal to the angle of incidence and therefore the projection angle. Since the projection angle typically varies from 0 to 57 degrees, the angle of reflection typically varies from 0 to 57 degrees. This is problematic in that it prohibits the use of screens with optical gain.
The use of screens with high optical gain is highly desirable as these screens afford improved brightness and contrast ratio for the visual display. Unfortunately, the high optical gain is in the specular reflection direction. As shown in FIG. 1, the specular reflection direction is away from the pilot's location for large projection angles. At large projection angles, the visual display brightness using a gain screen is less than if a standard Lambertian screen was used. (A Lambertian screen has an optical gain that is uniform in all directions but the gain is always one or less.)
Because of the integrating sphere effects, the gain should be less than one and typically, 0.5.+-.0.1. Thus, instead of using a screen with a gain of 4, which is typical for flat screen displays, dome visual displays typically must use a screen with a gain of only 0.5. Consequently, the brightness is only 1/8th as bright as the equivalent flat gain screen visual display.
Holographic screens were developed for use in simulators to reduce specular reflection thereby increasing the brightness of the image seen by the simulator pilot. A hologram has the unique characteristic that if light is incident on the hologram from one direction then light is caused to be propagated in a second direction other than the specular reflection direction. FIG. 2 shows a closeup of a holographic screen. As shown, the projected beam causes light to propagate in the direction of the simulator pilot. Depending upon the design and manufacture of the hologram, practically all of the light is propagated in the direction of the simulator pilot and ideally none of the light is propagated in the specular reflection direction. This enables high brightness visual displays because most of the projector light is propagated in the direction of the simulator pilot.
Initially, holographic projection screens were made using a diffuser to enable the simulator pilot to see the visual display. If the holographic projection screen is made without a diffuser, the pilot would see only a single bright point of light throughout the entire angular subtense of the holographic screen. For example, if the holographic screen were made in increments of one square foot, the simulator pilot would see only one bright point of light per square foot of holographic projection screen area.
The undesirable aspect of a holographic projection screen made with a diffuser is that it reproduces the speckle of the diffuser. Speckle is a phenomenon that occurs whenever coherent light is used to illuminate a diffuse surface. It appears as a grainy texture superimposed on the diffuse surface, but yet projected out in space to the plane of the observer and therefore it can be quite irritating to the observer. It is therefore desirable to eliminate the speckle associated with conventional holographic projection screens.
There is no speckle when coherent light is reflected from a smooth surface like a mirror. In that case, a spherical wave-front is produced without any of the interference which causes the speckle. The problem of a "single bright point per hologram" was eliminated by making each hologram smaller than the resolution of the visual display. This type of holographic projection screen is called a "microdot" holographic projection screen. Each hologram is smaller than the resolution of the visual display. Each hologram is essentially a high resolution picture of the interference pattern created by the interaction of two laser beams, a signal beam containing the "image" and a reference beam. When the two beams interact, the beams interfere with each other constructively and destructively. Where the beams interfere constructively, an area of maximum optical intensity is created and recorded on photographic film as a light area, typically a line. Likewise, where the two beams interfere destructively, an area of minimum optical intensity is created which is recorded on film as a dark area. When the photograph of the interference pattern thus created is illuminated by the reference beam, the input image is created.
Creation of the hologram has heretofore been a slow and cumbersome process due to the requirement that the film be held still while the interference pattern is recorded thereon. For example, U.S. Pat. No. 4,500,163, issued Feb. 19, 1985, to R. H. Burns et al., describes a step and repeat method for making microdot holographic projection screen holograms. However, making microdot holograms one hologram at a time is a slow process. The holographic film is mounted on an x-y transport. After moving to the center of the next microdot hologram, the x-y transport must stop moving (settle) before the exposure can begin. Mechanical movements of a fraction of a wavelength of light will ruin the interference pattern and hence the microdot hologram. Hence, the process is quite slow.
The following is an example of the length of time required to make a one square foot hologram with microdot hologram spacing at 8.29 mil intervals (2.1 million microdots per square foot).
______________________________________ Microdots per sec. Inches per second Time (hrs.) ______________________________________ 12 0.1 48.5 36 0.3 16.2 60 0.5 9.7 120 1.0 4.9 ______________________________________
Thus, the time required to make a square foot hologram could be several days.
Further, the start and stop movement requires considerable laser power because the photographic film may only be exposed after it has stopped moving. Power is consumed while the laser waits in a power up standby mode for the film to stop moving.
Thus, there is a need in the art for a faster technique for making microdot holograms which, ideally, also consumes less power.